1. Field of the Invention
The present invention relates primarily to the field of paper making machinery, and more particularly to an apparatus for applying variable amounts of steam to calenderable sheet material, thereby controlling certain properties of the material such as its cross-directional gloss profile.
2. Related Art
One of the parameters used in grading sheet materials is the gloss of the surface. In the paper production process, various grades of paper having different surface gloss are produced to suit various applications. Typically, bulk paper is produced in a continuous sheet and wound in rolls having dimensions 12 to 36 feet in the cross-direction (i.e., across the width of the sheet). Uniformity of gloss on the paper surface is often desirable or necessary. For example, in the situation where the roll of paper is cut to size for making various paper products, the consistency of the gloss of the individual paper items is dependent upon the uniformity of the gloss of the original bulk paper roll.
Paper production typically involves a calendering process which includes pressing paper material between calender rolls to obtain desired physical characteristics Calendering paper can change its density, thickness, and surface characteristics, including gloss In conjunction with calendering, steam is frequently applied to paper being calendered so as to moisten and heat the paper and thereby affect certain of its characteristics. For example, gloss is typically created on the surface of paper by applying steam to the paper surface, followed by pressing the paper between a series of calender rolls, typically arranged in a stack of alternating hard polished steel rolls and soft or resilient rolls made of cotton. The paper absorbs the steam, and paper fibers at the surface are softened by the heat and moisture. As the steam-treated paper surface comes into contact with the calender rolls, it is smoothed by the pressing and rubbing actions of the polished steel roll and adjacent cooperating soft roll. The degree of gloss created is dependent on the amount of moisture and heat and hence the amount of steam applied to the surface. Typically, very small amounts of steam are needed to create the desired gloss.
A common problem encountered in making a glossy finish using a steam treatment is the non-uniformity of the gloss finish of the calendered material. Localized variations in the amount of steam applied to the surface of the bulk paper may affect the uniformity of the gloss finish. Also, there are other variables in the calendering process such as temperature and calender roll pressure which may affect the amount of steam required for a particular degree of gloss. A more uniform gloss finish can be obtained if the amount of steam directed at different sections of the paper surface can be controlled.
Another common problem associated with the application of steam in calendering a gloss finish is that excess steam that has not been absorbed by the paper condenses on cool surfaces of the adjacent structure of the calender system. For example, the steam may condense on the steel calender roll, which will wet the paper as the steel roll contacts the paper. The extra moisture from the steel calender roll in addition to the moisture applied directly to the sheet from the steam supply will affect the moisture distribution and hence the gloss finish and other physical properties of the paper. In addition, excess steam may condense on a cool portion of the paper surface at a location where steam treatment is not intended, thereby affecting the gloss profile or causing "picking" (bulging and bubbling) when the paper becomes too wet. Moreover, steam which condenses on cool surfaces forms water droplets which may drip on the paper as it passes through the system of calender rolls, thereby again affecting the desired properties of the paper. Also, since the creation of gloss on a sheet of paper is generally the final step in paper manufacturing, the paper may already have been coated with various materials and an application of too much steam may cause problems by softening any water soluble coatings which then adhere to the calender rolls, causing "scaling".
In the past, steam boxes have been utilized to distribute steam on paper sheets during calendering. A steam box designed for applying steam to achieve uniform cross-directional gloss is disclosed in U.S. Pat. No. 4,786,529 to Boissevain. The disclosed steam box functions by discharging jets of steam through holes in the steam box. However, since more steam is discharged than can be absorbed by the paper, the excess steam must be eliminated to prevent its condensation on structures adjacent the steam treatment zone. To remove excess steam, a suction device is provided. Although highly useful in many situations, the steam box disclosed in U.S. Pat. No. 4,786,529 may not be appropriate in some situations because of its relatively complicated structure.
In an alternative design, a concentric pipe system has been utilized to distribute steam on a calenderable material to achieve certain properties such as gloss. One pipe system comprises an inner pipe having a top slot extending the length of the pipe. The inner pipe is surrounded by a second pipe having a slightly larger diameter and a bottom slot extending its length. The second pipe, in turn, is surrounded by yet a third pipe having a slightly larger diameter than the second pipe and a top slot extending its length. Steam flows first into the inner pipe, out its top slot into the second pipe, out of the bottom slot of the second pipe into the third pipe, and finally out of the top slot of the third pipe. A shortcoming of the concentric pipe system, however, is that it provides no control over the amount of steam applied to different sections of the paper surface; instead, this rather complicated structure provides a uniform amount of steam to all sections.